Make the #error argument a string
Make the #error argument a string

In the generated C++ source code, in the #error cases in the generated
checks, make the text a single string constant.

This follows the advice in the gcc documentation: "It is wisest to make
the argument of these directives be a single string constant; this
avoids problems with apostrophes and the like."

olgeni/gpb,mmmries/gpb